Optical fibres commonly comprises a glass core (typically with a diameter of about 120-130 μm), inside which the transmitted optical signal is confined, surrounded by a cladding, preferably made of glass. The combination of core and cladding is usually identified as “optical waveguide”. The optical waveguide is generally protected by one or more outer coatings, typically of polymeric material. In the production of optical fibres, a polymeric coating is applied immediately after drawing of the optical waveguide for protection and reinforcement of the optical waveguide. Generally, two coatings are applied, a first soft coating layer (known as primary coating or inner primary coating) of a flexible polymer (low modulus, typically of from 1 MPa to 2 MPa at room temperature, and low Tg) which is provided directly on the glass surface, and a second coating layer (known as secondary coating or outer primary coating) of a more rigid polymer (higher modulus, typically of from 500 MPa to 2000 MPa at room temperature, and higher Tg) which is provided over the primary coating layer. The overall diameter of the optical waveguide and the primary and secondary coating can be of from 150 to 250 μm.
A buffer coating can be further provided over the primary coatings as protective layer in tight configuration, being applied in direct contact with the primary or, more frequently, the secondary coating. The buffer coating has a thickness such to bring the overall optical fibre diameter to a value of 600-1500 μm, and is generally made of a polymeric material, either thermoplastic or cured.
The individual fibres are generally combined in larger structures such as cables. Cables may comprise individual fibres or fibre ribbon structures. The optical fibre ribbon generally is made from 2, 4, 6, 8 or 12 optical fibres, in general arranged in a plane, and bonded together with a so-called matrix material. Several ribbons can be bundled (stacked) together using bundling materials.
Polymers that cure on exposure to radiation such as ultraviolet radiation are favoured in the industry, due to their fast cure, enabling the coated fibre to be produced at high speed. These radiation curable polymer compositions can make use of urethane oligomers having reactive terminal groups (such as acrylate or methacrylate groups) and a polymer backbone. Generally the compositions further comprise reactive diluents, photoinitiators and additives.
There are several properties which are commonly required for optical fibre coatings. Some of these requirements are: low water absorption, low extractables, maintenance of the desirable levels of properties such as modulus, elongation, Tg, and adhesion under aggressive aging conditions (over relatively long periods of time) including high temperatures and/or high humidities, immersion in water and chemical resistance.
The acrylate coatings for optical fibres known in the art are generally resistant up to 200° C. along the optical fibre lifespan. Applications requiring higher operating temperatures (over 200-250° C.) usually employ optical fibres with coatings other than acrylate, but these coatings entail manufacturing problems and/or cost increase.
Optical fibre coatings based on urethane acrylate oligomers are disclosed, for example, in US 2004/0048946. This document relates to a radiation-curable solvent-free coating composition which exhibits good yellowing resistance under aging conditions such as high temperature and/or high humidity (125° C./dry or 85° C./85% RH). In particular the radiation-curable solvent-free coating composition comprises:
(A) a radiation-curable urethane (meth)acrylate oligomer comprising an alkyd backbone,
(B) a reactive diluent,
(C) a photoinitiator, and optionally
(D) an additive.
The oligomer (A) is obtained by reacting an aromatic or aliphatic polyisocyanate, an hydroxy-terminated alkyd and a compound providing the reactive terminal groups, such as hydroxyalkylacrylate.
Reactive diluent (B) preferably has a molecular weight of not more than about 550. The reactive diluent system may comprise compounds such as: isobornylacrylate, laurylacrylate, 1,6-hexanediol-diacrylate, alkoxylated bisphenol A diacrylate. Isobornyl acrylate, ethoxylated bisphenol A diacrylate, and hexane diol diacrylate are particularly preferred.
The composite oligomer can be incorporated into outer primary coating.
U.S. Pat. No. 6,438,306 relates to a coated optical fibre comprising a cured coating having a relatively low yellowing. The compositions are designed for use as uncolored optical fibre secondary (or outer primary) coating. The radiation curable composition comprises:
(A) an oligomer,
(B) a reactive diluent, and
(C) a photoinitiator package of at least two free radical photoinitiators.
The oligomer (A) is a urethane acrylate oligomer derived from a polyol (e.g. a polyether diol obtained by ring-opening copolymerization of tetrahydrofuran) reacted with a an aromatic or aliphatic diisocyanate and hydroxyalkylacrylate (e.g. 2-hydroxy ethyl (meth)acrylate). The average molecular weight of the urethane acrylate is from about 1,200 to about 20,000.
The reactive diluents (B) are, e.g., polymerizable vinyl monomers, such as 1,6-hexanediol di(meth)acrylate and alkoxylated bisphenol A diacrylate.
WO 02/074849 relates to a liquid curable resin composition suitable as a coating material. In particular, curable resin composition suitable for a secondary coating material affected by heat and humidity to only a small degree can be obtained by using a urethane (meth)acrylate obtained using a specific diol as a diol component, specifically at least one diol component (A1) selected from the group consisting of polypropylene glycol with an average molecular weight of 300-5,000, a copolymer of propylene oxide and ethylene oxide with a number average molecular weight of 300-5,000 and a copolymer 15 of ethylene oxide and butylene oxide with a number average molecular weight of 300-5000.
In addition to diol (A1), other polyols (A2) may optionally be used. As examples of polyols used as the component (A2), polytetramethylene glycol is mentioned.
As examples of the diisocyanate 2,4-tolylene diisocyanate, isophorone diisocyanate, and methylenebis(4-cyclohexylisocyanate) are preferable.
As examples of the (meth)acrylate, 2-hydroxyethyl (meth)acrylate is particularly preferable. A polymerizable polyfunctional compound may be added to the liquid curable resin. As examples of polyfunctional compounds, 1,6-hexanediol di(meth)acrylate and, preferably, di(meth)acrylate of ethylene oxide addition product of bisphenol A, are used.